It actually depends if Orange have a roaming agreement with the operators on Aiya Napa. If Orange are an operator in Aiya Napa then yes you will be fine.
Phone Orange Uk and ask them.
I wouldnt take an iPhone away, get some cheapo Nokia, you are bound to get pissed and lose it or get it nicked.
Edit....2 operators, MTN and Vodaphone also one Virtual op using Vodaphone. Orange should have an agreement with them. Check though.